IV. Cell TransportA. Diffusion Through Cell Boundaries*In solution, particles move constantly,collide, and spread out randomly.Diffusion - process by whichmolecules move from highconcentration to lowerconcentration.-Once the concentration is the same throughout a system, equilibrium is reached, but molecular movement doesn’t stop.-Random movement lets substancesdiffuse across cell membranes withoutusing a cell’s energy, a process calledpassive transport .

B. OsmosisOsmosis – the diffusion ofwater through a selectivelypermeable membrane.

-Water will move from an areaof high water concentration toan area of low waterconcentration, Ex : if a beakerhas many sugar molecules onone side of a semi-permeablemembrane, water will movetowards the sugar until itreaches equilibrium.

Movement of water can create 3 cell conditions :

1. Hypertonic cells – cell’s environment has a higher concentration of solutes than inside the cell. “Hyper” meaning higher.

Result : Water rushes out of cell & intothe environment, causing the cell toshrivel (plasmolysis).2. Isotonic cells – cell’s environment has the same concentration of solutes as inside the cell. “Iso” meaning same.Result : Perfect equilibrium, with nomovement of water in or out of the cell.3. Hypotonic cells – cell’s environment has a lower concentration of solutes than inside the cell. “Hypo” meaning lower.Result : Water rushes into the cell, causing itto swell & experience lysis(explosion/breaking apart).

C. Facilitated Diffusion

Facilitated Diffusion –diffusion of certainmolecules across cellmembranes via proteinchannels, ex : glucosemoving into red blood cells.

-Is like a short-cut. Glucosecan’t diffuse across the lipidbilayer on its own, so itmoves through a proteinchannel instead (see pg. 209).

The process uses no energy.

D. Active Transport*Sometimes cells mustmove materials in theopposite direction,against a concentrationdifference. This is doneby active transport.Active transport –energy-using processthat moves materialsacross a cell membraneagainst a concentrationdifference.-Is done by transportproteins found in themembrane itself.

Active vs. Passive Transport

Types of Active Molecular Transport*Larger molecules are transported across the cellmembrane by 2 processes :

1. Exocytosis – packaging & removal of wastes from the cell. The membrane of a vacuole containing waste fuses with the cell membrane and forces the

contents out of the cell.

2. Endocytosis – the process of taking outside material into the cell by means of

infoldings, or pockets, of the cell membrane.

Overview of Endo & Exocytosis

2 Types of Endocytosis

A. Pinocytosis – “Cellular drinking”. Is when a cell takes in liquid from the surrounding environment.

B. Phagocytosis – “Cellular eating”. Is when a cell engulfs an outside food

particle or some other solid substance, ex ; amoebas & white blood cells.